The disks of such a hydrodynamic brake are generally provided along their confronting faces with annular grooves which are subdivided, by angularly equispaced vanes flush with the disk faces, into adjoining pockets which are peripherally coextensive on the two disks so that their two sets of vanes are aligned with each other in certain relative angular positions. The retardation of the rotor disk by the entry of hydraulic fluid into the brake chamber converts the kinetic energy, without any wear of the moving parts, into thermal energy which may be dissipated in a heat exchanger; see, for example, U.S. Pat. Nos. 3,423,134 and 3,482,659.
The release of the brake requires the draining of its chamber and should enable virtually unimpeded relative rotation of the disks. In practice, however, a residue of hydraulic fluid (referred to hereinafter, for convenience, as oil) will remain trapped in the pockets and, together with the air volume in the chamber, will resist such relative rotation. Attempts have been made to obviate this drawback by the use of axially movable baffles which in the idling state can be shifted toward the gap between the rotor and stator disks to arrest the retarding air circulation; see, for example, German printed specification No. 1,675,248. Such a construction, requiring a large number of baffles and associated actuating elements, is complicated and correspondingly expensive.